_Messrs. Swift and Son_ have a new series of objectives, semi-apochromatic and pan-aplanatic, most of which are excellent in quality and show increased flatness of field together with that of achromatism; the index of refraction in each series having been correctly determined together with exact radial focal distance, thus affording more available aperture. I may select for special commendation their 1/12-inch £5 5_s._ homogeneous immersion objective, which is in every way suitable for bacteriological work; its definition is very good, as is seen in a micro-photograph of podura scale, given further on. Their dry 1/6-inch can be had for £1 16_s._--a marvel of cheapness. Of their general series the most useful for histological work are the 1/2-inch, the 1/3-inch at £1 12_s._, and their 1/5-inch of numerical aperture 0·87 at £3. _Mr. Pillischer_, of Bond Street, has manufactured many excellent objectives. A fine homogeneous oil-immersion 1/12-inch numerical aperture 1·25 is worthy of special notice; it will be found suitable for bacteriological work; it has fine definition with a considerable amount of penetration. A more intelligent idea of the magnifying power of the objective combined with the eye-piece will be gained by consulting the table given below; precision in this respect has long been a desideratum with microscopists. Magnifying Powers of Eye-Pieces and Objectives. A TYPICAL AND INITIAL SELECTION OF POWERS OF EYE-PIECES CALCULATED FOR THE 10-INCH TUBE-LENGTH. HUYGHENIAN EYE-PIECES. NAME A B C D E F OF MAKER. 0 or No. 1 2 3 4 5 6 Baker 6 8 12 15 -- -- Diameters. Beck, R. & J. 4 8 15 20 25 not made. " Leitz 5 6 7 8 10 12 " " Powell & Lealand 5 7·5 10 20 40 " " Reichert 2·5 3·5 4 5 6·5 " " [24] Ross 3 8 12-1/2 20 25 40 " [25] Swift & Son 6 9 12 15 18 21 " Watson & Sons 4 6 8 10 12 15 " Zeiss 3 4 5·5 7 9 not made. " COMPENSATING EYE-PIECES FOR USE WITH APOCHROMATIC OBJECTIVES. Zeiss 2 4 8 12 18 27 Diameters. This may be taken as a typical set, further treated of among Eye-pieces. INITIAL POWERS OF OBJECTIVES CALCULATED FOR THE 10-INCH TUBE-LENGTH. This is ascertained by dividing the distance of distinct vision 10 inches by the focus of the objective, thus-- Focus-inches 4 3 2 1-1/2 1 2/3 1/2 4/10 1/4 1/5 1/6 1/8 1/12 Initial magnifying power 2·5 3·3 5 7·5 10 15 20 25 40 50 60 80 120 diameters.